A special technique taught by the Shotokan school of Karate in the Street Fighter universe. It's literal meaning is "wave motion fist," which is accurate as it is an attack in which you "throw" a wave of your own chi as a weapon. As this move has been passed down a few variations of it have been formed, each with its own unique traits.

Hadoken
The standard version, a blue fireball that is weak but has enough power to knock an opponent down. Used by Ryu, Ken, Sakura, and Gouki.
Shankunetsu Hadoken
A version of the Hadoken powered with the element of fire. A very fierce version of the attack that is stronger than a regular hadoken. The attack deals high amounts of damage but is slower due to the longer charge time required to power it up. Used by Ryu and Gouki,
Shinkuu Hadoken
A Hadoken that is powered with a larger amount of chi, thus dealing a high amount of damage to the opponent. This attack can be fatal if the opponent is in a weakened state or if they cannot properly handle damage from chi attacks. Used by Ryu and Sakura
Messatsu Go Hado
A version of the Hadoken tainted with the "Murderous Intent." Chi for this attack is driven from hate and anger, plus the chi is charged from both hands then brought together. The amount of power behind it usually causes it to be normally fatal. Used by Gouki.
Denjin Hadoken
A hadoken that is infused with the power of electricity. Although it is a very weak attack unless charged to its maxium ability. But even then, this attack does not deal high damage. Instead it stuns the opponent allowing the attacker to deal in damage to his stunned opponent. Also, it is the fastest of all the hadokens. Used by Ryu.